1. Field of the Invention
The present invention relates to a developing apparatus adapted for use in an image forming apparatus, e.g., an electrophotographic optical printing machine.
In a conventional developing apparatus used in an image forming apparatus, such as an electrophotographic optical printing machine, a casing of a developing unit is provided with a plurality of developing rollers. In the developing apparatus of this type, the developing rollers are arranged along the rotating direction of a photosensitive drum, so that a developer is supplied in a plurality of steps to an electrostatic latent image formed on the drum. More specifically, each time the latent image on the drum faces a developing region of each developing roller, the developer is fed from the roller to the latent image.
Having the developing rollers arranged in this manner, the developing apparatus can enjoy a satisfactory developing characteristic, i.e., good image density. If n number of rollers are used in the developing apparatus, then the rotating speed of each developing roller, for a predetermined image density, must only be equal to about 1/n of that of a developing roller which is used singly in another developing apparatus. Thus, good image density can be obtained even with use of low-speed developing rollers. Moreover, the centrifugal force to act on a toner which adheres to each of the n developing rollers of the multi-roller developing apparatus is equal to 1/n.sup.2 of that for the case of the roller of the single-roller developing apparatus. Thus, the apparatus having two or more developing rollers has an advantage over the single-roller apparatus in being less liable to cause centrifugal scattering of the toner.
In the multi-roller developing apparatus described above, however, the profile of a void portion, which is surrounded by one developing roller (first developing roller), another developing roller (second developing roller) subsequent thereto, and the photosensitive drum, is widened sharply from the proximate portion between the rollers toward the drum. Therefore, some of convective air in the void portion stagnates, and toner cloud is produced at the stagnant region. As a result, the toner adheres to those portions of the electrostatic latent image which should not be coved with toner, thus fogging the resulting copy image.